The present invention concerns signal test and measurement and pertains particularly to signal modulation compensation for a wavelength meter.
In a Michelson interferometer system, light from a fiber optic input is collimated and directed to the input of the interferometer. The input signal is split into two paths with a beam splitter. Both beams are then reflected by mirrors that bounce the light back toward the beam splitter. Part of the light reflected from the mirrors goes back toward the input beam. The other portion of the light is incident on a photodetector. Since there is no loss assumed in the interferometer, all of the light is directed to either the photodetector or the input beam.
One of the mirrors of the interferometer is stationary and one is movable. The movable mirror is movable to vary the length the beam travels before and after incidence with the movable mirror. As the mirror is moved, the amount of light reaching the photodetector will oscillate up and down because of constructive and destructive interference effects between the two paths of the interferometer. Through the analysis of these interference patterns, the wavelength of light can be calculated.
The beams of light can be analyzed in terms of light interfering as the path length in the interferometer changes. This is referred to as the fringe-counting description of wavelength meter operation. Alternately, if the movable mirror is moved at a constant rate, the frequency of the light in the beam is Doppler-frequency shifted. The Doppler detector then mixes the light from the moveable mirror and the stationary mirror. The beat frequency between these two signals can be used to calculate the unknown frequency of the input signal. See Dennis Derickson, Fiber Optic Test and Measurement, Prentice Hall, Inc., 1998, pp. 133-141.
A Michelson interferometer based wavelength meter measures the wavelength of an unknown signal by comparing the fringe pattern produced by the unknown signal with that of the reference (known) signal. As one arm of the Michelson interferometer is translated (i.e., the mirror is moved) the interference pattern at the photodetector oscillates between high and low irradiance. Comparing the number of fringes produced by the unknown signal with the number produced by the known signal results in a highly accurate estimate of the unknown wavelength.
Unfortunately, if the unknown signal is amplitude modulated it becomes difficult to accurately count the number of unknown fringes. If Fourier transform techniques are used to determine the power spectrum of the signal the modulation produces false peaks equally spaced on either side of the true frequency called sidebands.
In accordance with the preferred embodiment of the present invention, an interferometer system is used to detect a wavelength of an unknown signal. The interferometer system includes a fringe pattern detection system and a power detecting system. The fringe pattern detection system measures an interference fringe pattern of the unknown signal. The power detecting system measures relative power of the unknown signal before the unknown signal enters the fringe pattern detection system. The relative power of the unknown signal is used to compensate for modulation within the unknown signal.